In an alternator producing 3-phase power, then one group of coils is at peak current while the others are not. Therefore the magnets align with only one phase at a time. Instead of figuring out how this is done from scratch, here’s the trick: For every coil of wire in the 3-phase stator, there are 1.33 magnets.

The absolute minimum number of coils in a 3-phase alternator is 3 coils. One for each phase. You would therefore need 4 magnets. Actually, that would be fairly clunky.
Here are some typical combinations,
Anything with more than 24 magnets is getting complicated, and the first-time builder should beware.
Similarly, varying the proportion of magnets and coils begs trouble, unless you know
how to avoid the pitfalls of making single-phase alternators (but you wouldn’t be a newbie).